The present invention relates generally to application of nickel-titanium alloys to medical devices. More precisely, the present invention is directed to cold worked nickel-titanium alloys that have pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device deployed from a sheath.
Near equi-atomic binary nickel-titanium alloys (nitinol) are known to exhibit xe2x80x9cpseudoelasticxe2x80x9d behavior when given certain cold working processes or cold working and heat treatment processes following hot working. Generally speaking, xe2x80x9cpseudoelasticityxe2x80x9d is the capacity of the nickel-titanium alloy to undergo large elastic strains on the order of 8 percent or more when stressed and to substantially fully recover all strain upon removal of the stress. Substantially full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.
Pseudoelasticity can be further divided into two subcategories: xe2x80x9clinearxe2x80x9d pseudoelasticity and xe2x80x9cnon-linearxe2x80x9d pseudoelasticity. xe2x80x9cNon-linearxe2x80x9d pseudoelasticity is sometimes used by those in the industry synonymously with xe2x80x9csuperelasticity.xe2x80x9d
Linear pseudoelasticity results from cold working only. Non-linear pseudoelasticity results from cold working and subsequent heat treatment. Non-linear pseudoelasticity, in its idealized state, exhibits a relatively flat loading plateau in which a large amount of recoverable strain is possible with very little increase in stress. This flat plateau can be seen in the stress-strain hysteresis curve of the alloy. Linear pseudoelasticity exhibits no such flat plateau. Non-linear pseudoelasticity is known to occur due to a reversible phase transformation from austenite to martensite, the latter more precisely called xe2x80x9cstress-induced martensitexe2x80x9d (SIM). Linear pseudoelasticity has no such phase transformation associated with it. Further discussions of linear pseudoelasticity can be found in, for example, T. W. Duerig, et al., xe2x80x9cLinear Superelasticity in Cold-Worked Nixe2x80x94Ti,xe2x80x9d Engineering aspects of Shape Memory Alloys, pp.414-19 (1990).
Because of the useful nature of the nickel-titanium alloy, some have attempted to change its properties to solve different design needs. For example, U.S. Pat. No. 6,106,642 to DiCarlo et al. discloses annealing nitinol to achieve improved ductility and other mechanical properties. U.S. Pat. No. 5,876,434 to Flomenblit et al. teaches annealing and deforming nitinol alloy to obtain different stress-strain relationships.
Binary nickel-titanium alloys have been used in the medical field. Many medical device related applications exploit the non-linear pseudoelastic capabilities of nitinol. Examples include: U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 to Jervis; and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746; 5,720,754; and 6,004,629 to Middleman, et al.
Yet another application of nickel-titanium alloys is in an embolic protection or filtering device. Such embolic filtering devices and systems are particularly useful when performing balloon angioplasty, stenting procedures, laser angioplasty, or atherectomy in critical vessels, particularly in vessels such as the carotid arteries, where the release of embolic debris into the bloodstream can occlude the flow of oxygenated blood to the brain or other vital organs. Such an occlusion can cause devastating consequences to the patient. While the embolic protection devices and systems are particularly useful in carotid procedures, they are equally useful in conjunction with any vascular interventional procedure in which there is an embolic risk. An embolic protection device that uses superelastic nitinol recently released to the market by the Cordis Corporation is known as the ANGIOGUARD.
What has been needed and heretofore unavailable in the prior art is a medical device that exploits the benefits of linear pseudoelastic nitinol. With the use of linear pseudoelastic nitinol, the mechanical strength of the device is substantially greater per unit strain than a comparable device made of superelastic nitinol. Furthermore, smaller component parts such as struts can be used because of the greater storage of energy available in a linear pseudoelastic nitinol device.
The present invention is generally directed to cold worked nickel-titanium alloys that have linear pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device having a strut formed body deployed from a sheath.
In one preferred embodiment, the present invention is directed to a medical device for use in a body lumen comprising a body formed from struts, wherein the body includes a cold formed nickel-titanium alloy, and the nickel-titanium alloy is in a martensitic phase when the body is stressed into a first shape and also when the stress to the body is relieved to assume a second shape. The present invention further includes a sheath at least partially enveloping the body in its first shape. The sheath may be used to transport the device to a targeted location in the patient""s anatomy, to deploy the device, and to retrieve the device at the end of the procedure.
The raw nitinol for use in the present invention has been cold formed and is further cold worked to set the desired expanded shape. Furthermore, the cold forming and cold working occur below the recrystallization temperature of the nitinol alloy.
During its operation, the linear pseudoelastic nitinol device can be stressed without developing stress-induced martensite in the alloy. Consistent with this behavior, an idealized stress-strain curve of the linear pseudoelastic nitinol does not contain any flat stress plateaus. Furthermore, despite application of stress, the nitinol alloy does not undergo a phase transformation from austenite to martensite or vice versa.
The resulting preferred embodiment device has greater mechanical strength at any given strain as compared to a device made of a standard superelastic nitinol. The stress-strain curve of the present invention linear pseudoelastic nitinol device also possesses more energy storage capacity. As a result, for a given desired performance requirement, the present invention linear pseudoelastic nitinol device allows for smaller struts and consequently a lower profile useful in crossing narrow lesions.
Another advantage is that because the present invention uses linear pseudoelastic nitinol, the underlying alloy can be selected from a broader range of available materials yet still maintain consistent, mechanical properties. In other words, there is less sensitivity to material variations and processing vagaries as compared to superelastic nitinol. In addition, since the linear pseudoelastic nitinol has no transformation from martensite to austenite or vice versa, there is less of an influence by temperature-related effects.
There are many specific applications for the present invention including vena cava filters, septal plugs, just to name a few. One specific application for the present invention is in a filtering device and system for capturing embolic debris in a blood vessel created during the performance of a therapeutic interventional procedure, such as a balloon angioplasty or stenting procedure, in order to prevent the embolic debris from blocking blood vessels downstream from the interventional site. The devices and systems of the present invention are particularly useful while performing an interventional procedure in critical arteries, such as the carotid arteries, in which vital downstream blood vessels can easily become blocked with embolic debris, including the main blood vessels leading to the brain. When used in carotid procedures, the present invention minimizes the potential for a stroke occurring during the procedure. As a result, the present invention provides the physician with a higher degree of confidence that embolic debris is being properly collected and removed from the patient""s vasculature during the interventional procedure.
An embolic protection device and system made in accordance with the present invention preferably includes an expandable filter assembly which is affixed to the distal end of a cylindrical shaft, such as a guide wire. The filter assembly includes an expandable strut assembly preferably made from a linear pseudoelastic nitinol, and includes a number of outwardly biased and extending struts that are capable of self-expansion from a contracted or collapsed position to an expanded or deployed position within a patient""s vasculature. A filter element made from an embolic capturing media is attached to the expandable strut assembly. The filter element opens from a collapsed configuration to an expanded configuration via the movement of the expandable struts similar to that of an umbrella.
The present invention further contemplates a medical device for use in a body lumen comprising a tubular body formed from small diameter tubing, a plurality of struts formed from a large diameter tubing and disposed on the tubular body such that the struts project radially outward in an unstressed state, wherein the large diameter tubing includes a cold formed nickel-titanium alloy, and the nickel-titanium alloy is in a martensitic phase only regardless of stress applied to the alloy, and a sheath at least partially enveloping the body and restraining the struts in a compressed state for delivery and retrieval of the device to and from the body lumen.
With this embodiment, it is no longer necessary to fabricate an expanded strut assembly from a small tubing that is heat treated to the expanded state. Rather, the expanded strut assembly starts out as a large diameter tubing wherein the struts themselves are formed from a large diameter tubing and assembled inward to the desired embolic protection device shape. The struts are preferably laser cut from the large tubing and are joined to the small diameter tubing such that in their unconstrained and stable state, they project radially outward thereby accomplishing the same expanded state without need of heat treatment.
By using a large diameter, cold worked or strain hardened nickel-titanium hypotube in the assembly of the expanded strut assembly, the strain hardened nickel-titanium material has increased mechanical properties that allow for the design of thinner walled interventional devices. Processing the interventional devices from large diameter hypotube allows for greater design flexibility and the ability to create more intricate designs, because of the increased surface area of the large diameter nickel-titanium hypotube. Moreover, a thinner walled device presents a reduced overall crossing profile and further improves system trackability through a tortuous anatomy.